Avoid buffer over-read on null cipher AEAD
Avoid buffer over-read on null cipher AEAD

In the defined AEAD modes, SRTP packets must always be encrypted and
authenticated, but SRTCP packets may be only authenticated.  It's
possible, therefore, for us to end up in `srtp_protect_aead()` without
the `sec_serv_conf` bit being set.  We should just ignore this and
encrypt the RTP packet anyway.

What we are doing instead is encrypting the packet anyway, but setting
`enc_start` to NULL first.  This causes `aad_len` to underflow which
will cause us to over-read in `cipher_set_aad()`.

If we could get past that, we would try to read and write memory
starting at 0x0 down in `cipher_encrypt()`.

This commit causes us to not check the `sec_serv_conf` bit and never
set `enc_start` to NULL in `srtp_protect_aead()`.

`srtp_unprotect_aead()` does not contain a similar error.

persmule/libsrtp,cuijinquan/libsrtp,persmule/libsrtp,cuijinquan/libsrtp,ksophocleous/libsrtp,yarrcc/libsrtp-ios,cuijinquan/libsrtp,persmule/libsrtp,yarrcc/libsrtp-ios,yarrcc/libsrtp-ios,ksophocleous/libsrtp